I have the QNAP TS-569 Pro and haven't experienced this issue with it; however, where I have seen it is for a USB drive I have attached to my router, for which I just had to wait for a few seconds until it became available.
@mikolajek, I'm running the same Windows version as you and am also on DOpus 13.2. Could it be that the issue may be related to the timeout setting per this post?